Ion mobility ionization type instruments (IMS) are employed to detect trace levels of chemicals in gas media. In IMS techniques, vapors of analyte molecules are ionized at ambient conditions, formed into ion species, and then identified by their coefficient of mobility. Conventional IMS ion identification is conducted in moderate direct current (DC) electric fields. In differential ion mobility spectrometry (DMS), which is a derivative of ion mobility technology, ion separation occurs by control of trajectories when ions move under the superimposed effect of a gas stream and a strong asymmetric waveform alternating current (AC) electric field. IMS and DMS are able to operate at atmospheric pressure conditions and, therefore, do not require cumbersome and high-power-consuming gas evacuating (pumping) equipment, thereby lending some portability to these instruments. Handheld prototypes of highly sensitive gas analyzers have been built, and numerous iterations are currently available. Historically, ion mobility-based instruments have been adapted for military, homeland security and space investigation applications. Currently, ion mobility technology is more and more involved in many civilian applications, such as chemical and biological processes control applications, for medical diagnostic applications, and for environmental control applications.
Typically, radioactive ion sources are employed by ion mobility sensors to ionize chemicals in a vapor state. Such sources often employ steel foil that has been implanted with radioactive isotopes, such as americium (240Am), nickel (63Ni), or tritium (3H). These ion sources are very convenient for field applications because they don't require large amounts of power, are lightweight, and are stable in operation. They also can provide both polarity ion species of either positive or negative charged ion species from analyzed ions. Nevertheless, the storage, transportation, periodic certification, disposal and safety requirements for using radioactive sources are generally highly regulated, which significantly increases the cost of operation and narrows potential applications, especially in various civilian applications.
Currently available non-radioactive methods of ion generation include corona discharge, ultraviolet (UV) ionization, and radiofrequency (RF) discharge ionization, including electromagnetic induction and capacitive gas discharge (CGD) methods. Known methods of these techniques all include limitations. For example, corona discharge often is not stable in the long term and can contaminate the sample with metal ions or nitrogen oxides (NOX), thereby interfering with analytical results. UV ionization typically is a more straightforward method of generating ions, but typically is limited to low or only moderate ionization energies, thereby limiting the types of molecules that can be ionized. RF discharge ionization methods, which include AC circuits, such as inductive and capacitive discharge techniques, also have their limitations. For example, electromagnetic induction methods typically are limited to high-powered discharges, such as are employed during production of refractory materials, abrasive powders, and the like. Most available CGD methods generally do not allow for ion source parameter optimization, such as minimization of plasma temperature to eliminate formation of nitric oxide (NOx) and ozone (O3) species due to plasma chemistry. These species have high electron affinity and therefore can capture free electrons from plasma, thereby resulting in the presence in the reaction chamber of these undesirable ion species, and reduced (or even fully suppressed) sensitivity to detection of negative ion species derived from analyte molecules.
Therefore, a need exists for a method of ion generation that overcomes or minimizes the above-referenced problems.